package com.github.easycaptcha.utils;

import java.io.IOException;
import java.io.OutputStream;

/**
 * Encoder
 */
public class Encoder {
  static final int BITS = 12;
  static final int HSIZE = 5003; // 80% 占用率
  private static final int EOF = -1;
  int n_bits;         // number of bits/code
  int maxbits = BITS; // user settable max # bits/code
  int maxcode;        // maximum code, given n_bits
  int maxmaxcode = 1 << BITS; // should NEVER generate this code
  int[] htab = new int[HSIZE];
  int[] codetab = new int[HSIZE];
  int hsize = HSIZE; // for dynamic table sizing
  int free_ent = 0;  // first unused entry
  // block compression parameters -- after all codes are used up,
  // and compression rate changes, start over.
  boolean clear_flg = false;
  int g_init_bits;
  int ClearCode;
  int EOFCode;
  int cur_accum = 0;
  int cur_bits = 0;

  // Algorithm:  use open addressing double hashing (no chaining) on the
  // prefix code / next character combination.  We do a variant of Knuth's
  // algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
  // secondary probe.  Here, the modular division first probe is gives way
  // to a faster exclusive-or manipulation.  Also do block compression with
  // an adaptive reset, whereby the code table is cleared when the compression
  // ratio decreases, but after the table fills.  The variable-length output
  // codes are re-sized at this point, and a special CLEAR code is generated
  // for the decompressor.  Late addition:  construct the table according to
  // file size for noticeable speed improvement on small files.  Please direct
  // questions about this implementation to ames!jaw.
  int masks[] =
      {
          0x0000,
          0x0001,
          0x0003,
          0x0007,
          0x000F,
          0x001F,
          0x003F,
          0x007F,
          0x00FF,
          0x01FF,
          0x03FF,
          0x07FF,
          0x0FFF,
          0x1FFF,
          0x3FFF,
          0x7FFF,
          0xFFFF};
  // Number of characters so far in this 'packet'
  int a_count;
  // Define the storage for the packet accumulator
  byte[] accum = new byte[256];

  // output
  //
  // Output the given code.
  // Inputs:
  //      code:   A n_bits-bit integer.  If == -1, then EOF.  This assumes
  //              that n_bits =< wordsize - 1.
  // Outputs:
  //      Outputs code to the file.
  // Assumptions:
  //      Chars are 8 bits long.
  // Algorithm:
  //      Maintain a BITS character long buffer (so that 8 codes will
  // fit in it exactly).  Use the VAX insv instruction to insert each
  // code in turn.  When the buffer fills up empty it and start over.
  private int imgW, imgH;    // 图片的宽高
  private byte[] pixAry;
  private int initCodeSize;  // 验证码位数
  private int remaining;     // 剩余数量
  private int curPixel;      // 像素

  //----------------------------------------------------------------------------

  /**
   * @param width       宽度
   * @param height      高度
   * @param pixels      像素
   * @param color_depth 颜色
   */
  Encoder(int width, int height, byte[] pixels, int color_depth) {
    imgW = width;
    imgH = height;
    pixAry = pixels;
    initCodeSize = Math.max(2, color_depth);
  }

  // Add a character to the end of the current packet, and if it is 254
  // characters, flush the packet to disk.

  /**
   * @param c    字节
   * @param outs 输出流
   * @throws IOException IO异常
   */
  void char_out(byte c, OutputStream outs) throws IOException {
    accum[a_count++] = c;
    if (a_count >= 254)
      flush_char(outs);
  }

  // Clear out the hash table

  // table clear for block compress

  /**
   * @param outs 输出流
   * @throws IOException IO异常
   */
  void cl_block(OutputStream outs) throws IOException {
    cl_hash(hsize);
    free_ent = ClearCode + 2;
    clear_flg = true;

    output(ClearCode, outs);
  }

  // reset code table

  /**
   * @param hsize int
   */
  void cl_hash(int hsize) {
    for (int i = 0; i < hsize; ++i)
      htab[i] = -1;
  }

  /**
   * @param init_bits int
   * @param outs      输出流
   * @throws IOException IO异常
   */
  void compress(int init_bits, OutputStream outs) throws IOException {
    int fcode;
    int i /* = 0 */;
    int c;
    int ent;
    int disp;
    int hsize_reg;
    int hshift;

    // Set up the globals:  g_init_bits - initial number of bits
    g_init_bits = init_bits;

    // Set up the necessary values
    clear_flg = false;
    n_bits = g_init_bits;
    maxcode = MAXCODE(n_bits);

    ClearCode = 1 << (init_bits - 1);
    EOFCode = ClearCode + 1;
    free_ent = ClearCode + 2;

    a_count = 0; // clear packet

    ent = nextPixel();

    hshift = 0;
    for (fcode = hsize; fcode < 65536; fcode *= 2)
      ++hshift;
    hshift = 8 - hshift; // set hash code range bound

    hsize_reg = hsize;
    cl_hash(hsize_reg); // clear hash table

    output(ClearCode, outs);

    outer_loop:
    while ((c = nextPixel()) != EOF) {
      fcode = (c << maxbits) + ent;
      i = (c << hshift) ^ ent; // xor hashing

      if (htab[i] == fcode) {
        ent = codetab[i];
        continue;
      } else if (htab[i] >= 0) // non-empty slot
      {
        disp = hsize_reg - i; // secondary hash (after G. Knott)
        if (i == 0)
          disp = 1;
        do {
          if ((i -= disp) < 0)
            i += hsize_reg;

          if (htab[i] == fcode) {
            ent = codetab[i];
            continue outer_loop;
          }
        } while (htab[i] >= 0);
      }
      output(ent, outs);
      ent = c;
      if (free_ent < maxmaxcode) {
        codetab[i] = free_ent++; // code -> hashtable
        htab[i] = fcode;
      } else
        cl_block(outs);
    }
    // Put out the final code.
    output(ent, outs);
    output(EOFCode, outs);
  }

  //----------------------------------------------------------------------------

  /**
   * @param os 输出流
   * @throws IOException IO异常
   */
  void encode(OutputStream os) throws IOException {
    os.write(initCodeSize); // write "initial code size" byte

    remaining = imgW * imgH; // reset navigation variables
    curPixel = 0;

    compress(initCodeSize + 1, os); // compress and write the pixel data

    os.write(0); // write block terminator
  }

  // Flush the packet to disk, and reset the accumulator

  /**
   * @param outs 输出流
   * @throws IOException IO异常
   */
  void flush_char(OutputStream outs) throws IOException {
    if (a_count > 0) {
      outs.write(a_count);
      outs.write(accum, 0, a_count);
      a_count = 0;
    }
  }

  /**
   * @param n_bits int
   * @return int
   */
  final int MAXCODE(int n_bits) {
    return (1 << n_bits) - 1;
  }

  //----------------------------------------------------------------------------
  // Return the next pixel from the image
  //----------------------------------------------------------------------------

  /**
   * @return int
   */
  private int nextPixel() {
    if (remaining == 0)
      return EOF;

    --remaining;

    byte pix = pixAry[curPixel++];

    return pix & 0xff;
  }

  /**
   * @param code int
   * @param outs 输出流
   * @throws IOException IO异常
   */
  void output(int code, OutputStream outs) throws IOException {
    cur_accum &= masks[cur_bits];

    if (cur_bits > 0)
      cur_accum |= (code << cur_bits);
    else
      cur_accum = code;

    cur_bits += n_bits;

    while (cur_bits >= 8) {
      char_out((byte) (cur_accum & 0xff), outs);
      cur_accum >>= 8;
      cur_bits -= 8;
    }

    // If the next entry is going to be too big for the code size,
    // then increase it, if possible.
    if (free_ent > maxcode || clear_flg) {
      if (clear_flg) {
        maxcode = MAXCODE(n_bits = g_init_bits);
        clear_flg = false;
      } else {
        ++n_bits;
        if (n_bits == maxbits)
          maxcode = maxmaxcode;
        else
          maxcode = MAXCODE(n_bits);
      }
    }

    if (code == EOFCode) {
      // At EOF, write the rest of the buffer.
      while (cur_bits > 0) {
        char_out((byte) (cur_accum & 0xff), outs);
        cur_accum >>= 8;
        cur_bits -= 8;
      }

      flush_char(outs);
    }
  }
}
